cow parsnip vs pigweed

cow parsnip

noun
  • Any umbelliferous plant of genus Heracleum. 

pigweed

noun
  • Heracleum sphondylium 

  • Polygonum aviculare (common knotgrass) 

  • Amaranthus species 

  • Portulaca bicolor, of Australia 

  • Any of various weedy plants sometimes used as pig fodder 

  • Chenopodium album, white goosefoot 

How often have the words cow parsnip and pigweed occurred in a corpus of books? (source: Google Ngram Viewer )